1. Field of the Invention
The present invention relates to a method of and an apparatus for reading image which can be employed in a color process scanner of a flat bed type and the like, and more particularly, it relates to a technique to compensate a positional reading error caused in serially reading an original image for every scanning line pitch with linear image sensors.
2. Description of the Prior Art
In a color process scanner and the like, three linear image sensors are provided corresponding to red, green and blue light components included in an incident light from an original, so that red, green and blue components of the original image are read by the three sensors for each pixel, respectively. The color image data for the respective color components obtained by reading the original image with the linear image sensors, or another data obtained by correcting the color image data is employed to produce a record image, which may be either a color separation image or a reproduced color image.
Therefore, in such an image reader, it is important to adjust a relation between the image reading positions of respective linear image sensors. If the image reading positions are not properly adjusted to each other, color light components from different positions on the original are composed, to give a reproduced image having positional color mismatch.
One of the most fundamental technique to prevent the problem is shown in FIG. 1. An image reading light L from a reading line W on an original 1 is led to an optical color separator 3 through a lens 2, by which the light L is separated into color components to be given to linear image sensors 4a to 4c, respectively. In the method, however, since the light L is separated into the color components, the light flux reading each one of the linear image sensors is decreased. Further, the optical color separator 3 having high resolution should be employed, so that the size and fabrication cost of the image reader are increased.
One the other hand, another technique has been developed, in which the positional reading error is compensated through adjusting a relation between image data output timings of respective linear image sensors rather than through employing the optical color separator, so that the light flux of the image reading light is not decreased. For example, an image reader shown in FIG. 2 is so constructed that images on different reading lines W.sub.a to W.sub.c are read by linear image sensors 4a to 4c detecting different color components, respectively.
In this case, when a subscanning is achieved by moving an original 1 to the direction (-Y), an image information of a pixel P is first read by the linear image sensor 4a, and then serially read by the other linear image sensors 4b and 4c, in this order. The respective color component data of the pixel P are delivered from the linear image sensors 4a to 4c, respectively, in the different timings.
In order to compensate the timing difference, the color component data delivered from the linear image sensors 4a and 4b, which read the pixel P prior to the linear image sensor 4c, are given to buffer memories 5a and 5b. The color component data are delivered from the buffer memories 5a and 5b with delay times respectively, so that the color component data S.sub.a to S.sub.c corresponding to the common pixel, and therefore, corresponding to a common scanning line are simultaneously outputted.
In an image reader such as that of FIG. 2 having no optical color separator, instantaneous reading lines W.sub.a to W.sub.c of FIG. 3 detected by the linear image sensors 4a to 4c at a same time are separated from each other at intervals of a predetermined distance A. The valve of the distance A depends on the geometric relations between respective elements in the image reader and the magnification of the lens 2. On the other hand, the width d of scanning lines l.sub.0, l.sub.1, . . . is determined according to the reproducing magnification in image recording. When the linear image sensors 4a to 4c are constructed as CCD (charge coupled device) linear arrays, the "scanning lines" are defined as a strip region on the original scanned by the instantaneous reading lines W.sub.a to W.sub.c with the movement of the original 1 in the direction (-Y) for one charge storage period of CCD photocells. In FIG. 3 and FIG. 4B which will be explained later, the width of the scanning lines is illustrated exaggeratedly.
Accordingly, the relation between the distance A and the width d may be variable, and the distance A is not always integral multiples of the value of the width d. Especially, the value of the width d can be continuously varied, and a mismatch between the distance A and the width d is caused inevitably.
When such a mismatch is caused, the linear image sensor 4c serially reads the image informations on the scanning lines l.sub.0, l.sub.1, . . . of FIG. 3, for example, while the other linear image sensor 4b serially reads the image informations on the scanning lines l.sub.0, l.sub.1, . . . , which are in the positions deviated from the scanning lines l.sub.0, l.sub.1, . . . , since the respective read timings in the linear image sensors 4a to 4c are common to these sensors. Such a deviation is caused also in the linear image sensor 4a.
Therefore, even if the output timings of the color component data from the linear image sensors 4a to 4c are adjusted with the buffer memories 5a and 5b, an image data faithful to the original image cannot be obtained, since the scanning lines read by the respective image sensors are essentially different from each other.
The magnitude of the positional color mismatch caused by the situation indicated above is smaller than the scanning line pitch d, and it is almost negligible in an ordinary image recording. However, it is important to effectively prevent the influence caused by the scanning line deviation, when an extreme high resolution image recording is required or the image recording is intended for an original on which optical density or color tone is drastically changed in a certain region.